In recent years, the spread of image pick-up devices represented by digital video cameras, digital still cameras and the like, as well as cellular/mobile phones and Personal Digital Assistants (PDA's) as display devices for displaying images, text, and the like has been remarkable. Liquid Crystal Displays (LCD's) which are thin-shaped, lightweight with low-power consumption are commonly carried everywhere. Also, amid the rapid replacement of older conventional Cathode Ray Tube (CRT) monitors or displays of computer terminals, televisions and the like with spacesaving devices requiring less power than in the past and due to their excellent image display quality, LCD's are increasingly being manufactured for a multitude of useful purposes.
FIG. 12 is an outline block diagram showing an example of the configuration of the section concerning the output of the display signal voltage of the data driver as applied to a liquid crystal display in a conventional technology.
FIG. 13 is a characteristic drawing showing an example of the relationship of the output level to the input data of a data driver in conventional technology.
In a data driver of prior art, as shown in FIG. 12, for example, is constituted with the changeover switches SPA, SPB, a division resistance Rp, a digital-to-analog converter (D/A Converter: DAC) 10 and an output amplifier AMP 20. The changeover switch SPA is configured with the reference voltage VRH by the high potential side connected to contact Npa and the reference voltage VRL by the low potential side connected to Npb. The changeover switch SPB is configured with the reference voltage VRL by the low potential side connected to contact Npc and the reference voltage VRH by the high potential side connected to contact Npd. The reference voltage (either the high potential side reference voltage VRH or the low potential side reference voltage VRL) are supplied on one end side and on the other end side while selected by the changeover switches SPA and SPB. The division resistance Rp performs a plurality of voltage divisions of the potential difference between the reference voltages supplied to both ends. The D/A Converter DAC 10 to which a plurality of gradation voltages produced by the reference voltage and the division resistance Rp selected by the changeover switches SPA and SPB is supplied, the display data which is composed of digital signals is inputted, and the gradation voltages according to the luminosity gradation of the display data are selected and converted into analog voltage. The output amplifier AMP 20 supplies each of the data lines DL by converting the analog voltage into the display signal voltage Vsig. Here, the changeover switches SPA and SPB switch and control each contact based on a polarity changeover signal POL, which controls the signal polarity of the display signal voltage Vsig, and reverse control of the signal polarity of the display signal voltage Vsig is suitably performed.
In such a configuration, when the polarity changeover signal POL is a high level (“H”) as shown in FIG. 13, the changeover switch SPA switches and controls the contact Npa side, and the changeover switch SPB switches and controls the contact Npa side as luminosity gradations of the display data. When the digitized data 00h (the lowest gradation: corresponds to a black display) is inputted, the reference voltage VRH by the high potential side is outputted as the lowest gradation voltage of the display signal voltage Vsig. When the digitized data 3Fh (the highest gradation: corresponds to a white display) is inputted, the reference voltage VRL by the low potential side is outputted as the highest gradation voltage of the display signal Vsig. Also, when the display data of the middle gradations is inputted, the gradation voltage corresponding to the gradation data of the display data is outputted as the display signal voltage Vsig from a plurality of gradation voltages produced by the division resistance Rp.
Conversely, when the polarity changeover signal is a low level (“L”), the changeover switch SPA switches and controls contact Npb side, and the changeover switch SPB switches and controls the contact Npd side. Accordingly, such as the characteristic curve of POL=“L” as shown in FIG. 13, when digitized data 00h (the lowest gradation) is inputted as the luminosity gradation of the display data, the reference voltage VRL by the low potential side is outputted as the lowest gradation of the gradation voltage of the display signal voltage Vsig. When the digitized data 3Fh (the highest gradation) is inputted, the reference voltage VRH by the high potential side is outputted as the highest gradation voltage of the display signal voltage Vsig.
Subsequently, the write-in operation of the display signal voltage to the display pixels of an active-matrix type liquid crystal display panel will be briefly explained.
FIG. 14A is an equivalent circuit drawing showing the configuration of the display pixels in an active-matrix type liquid crystal display panel.
FIG. 14B is drawing showing the drive voltage waveform in the case of writing display signal voltage to the display pixel clusters of a predetermined line of the liquid crystal display panel.
The display pixels Px in an active-matrix type liquid crystal display panel, as shown in FIG. 14A, is comprised with a configuration which has a pixel transistor (Thin-Film Transistor) TFT, a liquid crystal capacity Clc and a storage capacitance Ccs. The Thin-Film Transistor TFT by which the source-drain (current path) are connected between the pixel electrode and the data line DL to constitute the liquid crystal capacity Clc, the gate (control terminal) is connected to the scanning line SL, and the single electrode (counter electrode) is arranged countered to the pixel electrode and this pixel electrode. The liquid crystal capacity Clc consists of liquid crystal molecules filled between the counter electrode and the pixel electrode. The storage capacitance Ccs which maintains the signal voltage applied to the liquid crystal capacity Clc (for example, a common signal voltage Vcom) is constituted in parallel with this liquid crystal capacity Clc and connected on the other end side to the predetermined voltage Vcs.
The driver voltage waveform shown in FIG. 14B illustrates a case application of a field reversal drive method which drives the display signal voltage of positive and negative polarity so that it is written to each of the display pixels Px at 30 Hertz (Hz). Therefore, one screen is rewritten every one 60 Hz field period and controlled so that the signal polarity of the display signal voltage is reversed in every one field period. Specifically, the display signal voltage Vsig corresponding to the display data is applied to the pixel transistor TFT drain electrode via the data lines DL in every one field period. Here, the display signal voltage Vsig is set so that the signal polarity alternately reverses to the predetermined center level (display signal center voltage) Vsigc for every one field period. As in FIG. 14B, the display signal voltage Vsig of positive polarity is applied in the n-th field and the display signal voltage Vsig of negative polarity is applied to the n-th +1 field.
Conversely, only during the predetermined write interval (write-in period) Tw of the applied period of the above-mentioned display signal voltage Vsig, the scanning signal Vg is applied to the gate electrode of the pixel transistor TFT via each of the scanning lines SL, and the pixel transistor TFT performs an “ON” operation. Accordingly, the display signal voltage Vsig currently applied to the drain electrode is applied to the pixel electrode connected to the source electrode side. The display signal voltage Vsig is maintained as the pixel electrode voltage Vp until the write-in interval Tw in the next field by the storage capacity Ccs, while the liquid crystal molecules filled between the common electrodes are controlled in a predetermined orientation state. Moreover, the common signal voltage Vcom alternately reverses polarity to the predetermined center level Vcomc in every one field period.
Incidentally, in the liquid crystal display which employs the active-matrix type drive system mentioned above, as shown in FIG. 14B, in the case where the pixel transistor TFT switches from an “ON” state to an “OFF” state according to the applied state of the scanning signal Vg, it is recognized that the so-called “field through phenomenon” originating in the charge accumulated in the liquid crystal capacity Clc, the storage capacitance Ccs and the parasitic capacitance Cgs between the gate-source is redistributed, and that changes to the electrode voltage Vp will occur. Here, generally the fluctuation (field through voltage) ΔV of the pixel electrode voltage Vp by the field through phenomenon is expressed with the following formula (1):ΔV=Cgs×Vg/(Cgs+Clc+Cs)  (1)
Because such field through voltage ΔV generates the electrode voltage Vp in the direction that habitually makes it decrease at the time the scanning signal Vg drops as shown in FIG. 14B, it will change to the negative voltage side of the display signal voltage Vsig positive-negative signal polarity, and the pixel electrode voltage Vp becomes asymmetrical to the center level Vsigc of the display signal voltage Vsig. Therefore, the direct current voltage component on the voltage applied to the liquid crystal capacity Clc resulting from the difference (offset potential) of the positive-negative voltage of the pixel electrode voltage Vp to the center level Vsigc of the display signal voltage Vsig occurs. This represents the origin which causes characteristic deterioration of the display panel accompanying the generation of flicker or accompanying the sticking of the liquid crystal molecules.
Then, in order to control such fault in the past, as shown in FIG. 14B, generally the method of controlling or canceling the imbalance of the pixel electrode voltage Vp positive-negative polarity to the common signal voltage Vcom employed is by compensating (ΔV correction) only the above-mentioned offset potential to the center level Vsigc of the display signal voltage Vsig of the center voltage (common signal center voltage) Vcomc applied to the common electrode.
Here, the relationship between the applied voltage to the liquid crystal and the field through voltage ΔV will be explained.
FIGS. 15A, 15B and 15C are characteristic drawings showing the relationship of the applied voltage to the liquid crystal with the liquid crystal dielectric constant, the liquid crystal capacity and the field through voltage, respectively.
The liquid crystal capacity Clc has the relationships of the following formula (2) to the liquid crystal dielectric constant e (epsilon or “e”), the area S of the pixel electrode and the cell gap d. As shown in FIG. 15A, the dielectric constant e has the characteristic of changing to applied voltage V. As shown in FIG. 15B, the liquid crystal capacity Clc has the change inclination equivalent to the liquid crystal dielectric constant e to the applied voltage V.Clc=e×S/d  (2)
Here, since as the field through voltage ΔV has the relationship depending on the change of the liquid crystal capacity Clc as shown in the above-mentioned formula (1), the field through voltage ΔV has the characteristic of complexly changing to the applied voltage V (namely, display signal voltage Vsig) as shown in FIG. 15C. (Hereinafter, description of the change characteristic to applied voltage V of the field through voltage ΔV will be referred to as “ΔΔV characteristic” for convenience.)
However, in the past as shown in FIG. 13, the center level (display signal center voltage) Vsigc in reverse signal polarity of the display signal voltage Vsig (gradation voltage) is set so that it becomes a constant value to the input data (luminosity gradation). Therefore, as shown in FIG. 14B, by the method compensated only by a constant offset potential which previously set the common signal voltage Vcom, it migrates the overall gradation range of the display signal voltage Vsig. The fluctuation of the pixel electrode voltage Vp by the field through voltage ΔV can not be canceled favorably, and the generation of flicker under the effect of the field through voltage ΔV, sticking of the liquid crystal molecules and the like cannot be sufficiently controlled.